1. Field of the Invention
The present invention relates to the production of a printing plate from a liquid photosensitive resin. More particularly, the present invention is concerned with an improved method for selectively curing a liquid photosensitive resin by masking exposure in the production of a printing plate, particularly a flexographic printing plate. The printing plate produced is used in printing a paper container, a corrugated board, a film or the like.
2. Discussion of the Related Art
For the printing of a paper container, a film, a corrugated board or the like by letterpress printing, rubber plates and photoresin plates are used. A photoresin printing plate may be prepared either by processing a solid plate made of a cast photosensitive resin or by forming a layer of a liquid photosensitive resin and exposing the formed resin layer to active rays followed by such steps as removal of the uncured resin, to thereby obtain a printing plate. Incidentally, in letterpress printing such as flexographic printing, the printing plate is required to have a thickness of from 3 to 8.5 mm, for compliance with the mechanical requirements of the printing machine. In the above-mentioned process for preparing a printing plate from a liquid photosensitive resin, in order to produce a printing plate having the large thickness as mentioned above, having a relief which has a large depth and, at the same time, is capable of reproducing fine patterns and nevertheless does not bend or break by the printing pressure, it has been proposed to prepare the printing plate by process steps in which the uncured resin layer is irradiated with active rays from the side opposite to the relief side through a masking film having a transmitting pattern having a contour which corresponds to and is slightly larger than that of the transmitting pattern of an image bearing transparency, to thereby form a shelf portion for supporting a relief portion (this method is the so-called "masking exposure").
Referring to FIGS. 1 to 4, a brief explanation is provided below with respect to an example of the process for preparing a printing plate in which the masking exposure is involved. FIG. 1 is a diagrammatic cross-sectional view of a laminated structure which is to be subjected to masking, relief and back exposures, FIG. 2 is a diagrammatic cross-sectional illustration showing an arrangement employed for effecting the masking exposure and the relief exposure, and FIG. 3 is a diagrammatic cross-sectional illustration showing an arrangement employed for effecting the back exposure.
(1) Lamination [FIG. 1]
Image bearing transparency 2 is disposed on lower rigid plate 1 transparent to active rays, such as a glass plate, which in turn is covered with thin transparent protecting film 3. On protective film covering image bearing transparency 2, a liquid photosensitive resin is poured. Transparent substrate 5, such as a polyester film having a thickness of 0.1 to 0.2 mm, is laminated on the poured resin by a roll laminating method, while simultaneously leveling the resin into layer 4 with a predetermined thickness by applying a pressure to the resin by means of the roller used in the laminating method.
On substrate 5 laminated on liquid photosensitive resin layer 4 having the predetermined thickness, is superposed masking film 6 having a transmitting pattern having a contour which corresponds to and is slightly larger than that of the transmitting pattern of the image bearing transparency so that the position of the transmitting pattern of the masking film is in correspondence with the position of the transmitting pattern of the image bearing transparency. The masking film comprises a transparent film generally having a thickness of 75 to 200 .mu.m with a light-shielding thin film disposed thereon, wherein portions of the thin film which correspond to the desired pattern are cut out to form a transmitting pattern.
(2) Masking exposure [FIG. 2]
An upper light source box positioned above substrate 5 which contains upper light source 7 and whose bottom opening has upper rigid plate 17 which is transparent to the active rays, such as a glass plate, and extends over the bottom opening, is pulled down, so that upper rigid plate 17 is superposed on the masking film. The lower surface of upper rigid plate 17, which faces the masking film, is in a roughened state and has groove 20 provided thereon along the periphery. Groove 20 and the clearance formed between the roughened lower surface of upper rigid plate 17 and masking film 6 is evacuated in order to attract masking film 6 and substrate 5 toward upper rigid plate 17, causing the masking film and the substrate to be pressed against the upper rigid plate. This procedure is employed in order to secure the flatness of each of masking film 6 and substrate 5, thereby serving to produce a printing plate having a uniform thickness and which is free from dents and bumps. The resin layer is then exposed to active rays, such as ultraviolet rays, through the transmitting pattern of masking film 6, thereby selectively curing the resin layer to form shelf portion 9 which is to be a support for relief portion 16 which is later formed.
(3) Relief exposure [FIG. 2]
After the masking exposure, the resin layer is exposed to active rays which are emitted from lower light source 10 through the transmitting pattern (image pattern) of the image bearing transparency to form relief portion 16.
(4) Back exposure [FIG. 3]
After the relief exposure, the upper light source box is lifted and masking film 6 is removed. Then the upper light source box is lowered, and the resin layer is exposed for a short period of time to active rays which are emitted from upper light source 7 to form thin cured resin layer 11 having a uniform thickness in the resin layer over the entire area facing substrate 5. Thin cured resin layer 11 thus formed by the back exposure will be hereinafter referred to as "back layer".
(5) Removal of the uncured resin
After the back exposure, the uncured resin is recovered.
(6) Washing
The uncured resin remaining is washed out by nozzle washing or brush washing using a wash-out solution, thereby obtaining washed-out plate 14 as shown in FIG. 4.
(7) Post-exposure in water
The thus obtained plate 14 is completely immersed in water and exposed to active rays such as ultraviolet rays which are emitted from a light source to perform a complete curing of the entire plate so that the strength of the plate is increased.
(8) Drying
Plate 14 is removed from the water and dried by blowing the plate with hot air or by the use of an infrared heater.
In the process described above, the shelf portion, the relief portion and the back layer are formed in this order using a masking film having a transmitting pattern having a contour which corresponds to and is slightly larger than that of the transmitting pattern of the image bearing transparency. However, the shelf portion, the relief portion and the back layer may be formed in different ways. For example, the relief portion may be formed prior to or simultaneously with formation of the shelf portion. Further, there may alternatively be employed a method in which the shelf portion and the back layer are formed simultaneously by the use of a masking film provided with a transmitting pattern having a contour which corresponds to and is slightly larger than the pattern of the image bearing transparency, a semitransmitting pattern positioned outside the transmitting pattern and having a large contour for transmitting irradiated active rays upon attenuation, and a blocking surface positioned outside the semi-transmitting pattern and having an effect to block the active rays to an extent not to cure the liquid resin layer (see, for example, U.S. Pat. No. 4,618,550). In the method of U.S. Pat. No. 4,618,550, the masking film comprises a semilight-blocking first blocking film affixed to one surface of a transparent base film for passing active rays upon attenuation, and a second blocking film provided on the other surface of the base film for blocking active rays to such an extent that the rays will not cure the liquid resin used. The first and second blocking films are removably adhered to the base film with an adhesive. The second blocking film has properties to completely block light by itself, or is equal to or higher than the first blocking film in active attenuation ratio, such that when active rays pass through both the first and second blocking films, the rays are attenuated and blocked almost completely by the two blocking films. In the above method, when the laminate is exposed to active rays, the liquid resin layer is cured to a large thickness in corresponding relation to the transmitting patterns of the first and second blocking films to form a shelf portion integral with a relief portion. Where the light through the transmitting pattern of the second blocking film passes through the first blocking film, i.e. a semilight-blocking film and is thereby attenuated, the upper portion of the resin layer is cured to a small thickness in corresponding relation to the transmitting pattern of the second blocking film to form a back layer integral with the shelf portion. As mentioned above, in the method of U.S. Pat. No. 4,618,550, a shelf portion and a back layer are simultaneously formed by the masking exposure and therefore a back exposure is omitted. In the present invention, such masking exposure for forming both the shelf portion and back layer is also defined as "masking exposure".
The thus obtained printing plate 14 has such a construction as shown in FIG. 4. In FIG. 4, numeral 16 designates the relief portion which has been formed by the relief exposure, numeral 15 the surface of the relief portion on which a printing ink is to be applied, numeral 9 the shelf portion which has been formed by the masking exposure, numeral 11 the back layer which has been formed by the back exposure or has been formed simultaneously with formation of the shelf portion by the masking exposure (U.S. Pat. No. 4,618,550), and numeral 5 the substrate. Further, character T.sub.0 indicates the total thickness of printing plate 14, character T.sub.1 the thickness of the shelf portion and character T.sub.2 the thickness of the back layer. For a printing plate having a thickness (T.sub.0) of, e.g. 7.0 mm, T.sub.1 and T.sub.2 are generally in the ranges of 4.8 to 5.8 mm and 1.0 to 1.5 mm, respectively.
From the above-mentioned process, due to the formation of the shelf portion by the masking exposure, there can advantageously be obtained a printing plate having a large thickness and having a relief which has a large depth and, at the same time, is capable of reproducing fine patterns and nevertheless does not bend or break by the printing pressure. In addition, this process is advantageous in that the expensive photosensitive resin remaining uncured can be recovered with a high recovery ratio and, hence, the process is economical. Therefore, this method is widely practiced.
However, the conventional process for preparing a printing plate, in which a masking exposure is involved, has the following problem. The masking film used in the conventional process is composed of a transparent film, such as a polyester film, having smooth surfaces at both sides and a thickness of 0.075 mm to 0.125 mm and, coated on its one surface adapted to face the upper rigid plate to be superposed thereon, a light-shielding thin film wherein portions of the thin film which correspond to the desired pattern are cut out to form a transmitting pattern. That is, the masking film used in the conventional process has a smooth surface at the side facing the substrate. Further, the substrate used in the conventional process is a polyester film of 0.1 to 0.2 mm in thickness and having smooth surfaces at both sides. That is, the surface of the masking film facing the substrate and the surface of the substrate facing the masking film are both smooth. Therefore, as illustrated in FIG. 5 (a), when substrate 5, together with masking film 6, is attracted toward and pressed against upper rigid plate 17 by evacuation in order to secure the flatness of each of masking film 6 and substrate 5 in above-mentioned step (2), a local interfacial cohesion frequently occurs between masking film 6 and substrate 5, causing masking film 6 and substrate 5 to be maintained with air pocket 26 formed therebetween. This air pocket causes the substrate to protrude into the liquid photosensitive resin layer during the operations of masking, relief and back exposures and, therefore, a dent tends to be formed in the cured resin layer, leading to a non-uniformity of the thickness of the ultimate printing plate, i.e. a local decrease in the thickness of the ultimate printing plate. In order to solve this problem, there has been proposed a method comprising, in combination, effecting vacuum-attraction of the substrate toward upper rigid plate 17 intermittently and gradually increasing the vacuum degree in the evacuation for the vacuum-attraction of the substrate. However, it has not been possible to completely prevent the formation of such an air pocket and, thus, the occurrence of the local decrease in the thickness of a printing plate has not been avoided.